I think a better way of looking at things is does your G5 meet your requirements? Does Snow Leopard offer anything that would make you more productive to the point where a hardware upgrade would be a good idea?

It's likely the PowerPC G5 series are the last of the PowerPC machines to be supported, even if they can't support the full 64-bit address space.

The PowerPC 970 (G5 in Apple parlance) does support both 32-bit and 64-bit Power ISA, so there shouldn't be any technical reasons why they couldn't. Supporting both 32-bit and 64-bit is actually quite a bit simpler on the 970, as it is able to switch between those modes on the fly.

Plus, 10.5 all ready supports a 64-bit Power runtime for userland, and so 10.6 just seems to be bringing 64-bit to the kernel as well.